1. Field of the Invention
The present invention relates to an electron beam blanker, and in particular to a differential virtual ground beam blanker.
2. Description of the Related Art
Beam blanking is a required function for all electron beam systems used in lithography, testing, metrology, and inspection. Double deflection beam blankers are well known in the art, and are described, for example, in U.S. Pat. No. 4,445,041 issued Apr. 24, 1984 to Kelly et al. and U.S. Pat. No. 5,276,330 issued on Jan. 4, 1994 (hereinafter Gesley 330). Double deflection beam blankers reduce beam positional jitter at the target due to electric field effects which occur during blanking.
Beam positional jitter is minimized at the target by balancing the interaction of the blanker voltage and the electron transit time through the blanker. The double deflection beam blanker is configured as a matched impedance transmission line to minimize voltage reflections along the line, thereby reducing the electric field effects on the beam. The double deflection beam blanker reduces the beam transit time effect on positional accuracy at the substrate-target by producing a second, delayed impulse to the beam as it passes through the blanker assembly.
However, a number of electrodynamic effects misplace the beam during and after the blanking operation, thereby producing spurious exposures in lithography or inaccurate measurements during inspection. One of these electrodynamic effects is a magnetic field interaction referred to as eddy currents. Typically, a blanker driver current indirectly interacts with the beam and generates a time-dependent magnetic field. This field in turn induces currents in neighboring conductors, such as the blanker housing. Eddy currents in these conductors then produce another magnetic field, commonly referred to as a B-field, which subsequently interacts with the electron beam on a much longer time scale, i.e. having exponential decay behavior. Even though individual pixel exposure times are small for photolithographic applications, e.g. with 160 MHz data rates that have 6.25 nsec pixel dwell times, the induced time-dependent magnetic field generated by the eddy currents can have serious effects over relatively "long" times compared to the blanking transition, i.e. from microseconds to hundreds of milliseconds. Thus, these eddy currents undesirably influence the beam position at the target located below the blanker.
Thus, a need arises for a double-deflection beam blanker that reduces these spurious magnetic field effects, thereby resulting in a blanking system with higher beam positional accuracy.